Role of Manganese in Accelerating the Oxidation of Pb(II) Carbonate Solids to Pb(IV) Oxide at Drinking Water Conditions.

Pb(II) carbonate solids are corrosion products that form on the inner surfaces of lead service lines (LSLs) and can be oxidized by free chlorine to form Pb(IV) oxide (PbO2). The formation of PbO2 can maintain low dissolved lead concentrations in drinking water, but PbO2 can dissolve if a free chlorine residual is not maintained. Experiments demonstrated that the oxidation of Pb(II) carbonate by free chlorine was faster with manganese (Mn). Without Mn(II), the oxidation of Pb(II) carbonate was an autocatalytic process. With Mn(II), the overall oxidation rate was 2 orders of magnitude faster than without Mn(II). X-ray diffraction and free chlorine consumption profiles indicated that δ-MnO2 was formed within several minutes of the reaction of Mn(II) with free chlorine, and δ-MnO2 catalyzed the oxidation of Pb(II) carbonate by free chlorine. Free chlorine consumption profiles for Pb(II) carbonate with and without Mn(II) were interpreted based on the kinetics and stoichiometry of the underlying chemical reactions. These findings highlight the importance of Mn in accelerating the formation of PbO2 in water with Pb(II) carbonate solids and free chlorine, and it may help explain why PbO2 is observed on LSLs of some but not all water systems that use free chlorine.